The present invention relates to the field of alternators, and particularly to the field of controlling alternator output.
FIG. 1 shows a conventional alternating current generator, or "alternator" 10. The alternator comprises a rotor winding (the field winding) 12, three phase stator windings (delta or wye configured) 14, 16, and 18, and a three phase full wave controlled rectifier bridge 20 connected to the stator windings. The alternator is connected to a vehicle engine which turns a rotor (not shown) holding the field winding. Rotation of the field winding 12 causes AC power to be generated in each of the stator windings. Power generated in the stator windings is three phase power, represented by phase A, phase B, and phase C. Each phase is delivered to the full wave rectifier bridge 20 where it is converted into DC power for delivery to the vehicle load 15. A battery 30 is connected in parallel with outputs of the full wave rectifier bridge 20, for delivering adequate power to the load when the field winding is not rotating or when the field winding is rotating too slowly to result in a voltage equal to the battery voltage. When the field winding rotates at an increased speed, a voltage results across the battery terminals that is greater than the battery voltage, and the battery is re-charged.
When the vehicle engine is idling such that the field winding rotates at a decreased speed, the output of the alternator may not be adequate to supply all the power required by the load. When this occurs, the alternator is no longer regulating the system voltage. The battery is being discharged as it attempts to augment the generator output to meet the power demand at the loads. If this condition remains over an extended period of time the battery will become completely discharged.
The most common way to control the output of the alternator during engine idling and provide extra power for delivery to the vehicle load is to increase the rotor field flux. This may be accomplished by increasing the current through the field winding. If a greater current is delivered through the field winding, a greater voltage will be induced in the stator windings, and a greater output from the alternator will result as the engine idles and the rotor rotates at a slow speed. However, the amount of current that may be delivered to the field winding is limited by temperature concerns, as too much current flow through the field winding will cause the alternator to overheat.
Another effective method for obtaining an increased output from the alternator during engine idling is to maximize the power angle. The power angle is defined as the phase difference between the back EMF generated in the stator windings and the phase voltage output from the stator windings. FIG. 2 shows a schematic diagram of an equivalent circuit of a synchronous electromagnetic machine having a uniform air gap, wherein: E is the back EMF generated in one of the stator windings by a rotating flux produced by the field winding; I is the phase current; R is the resistance of the stator winding; X is the reactance of the stator winding; and V.sub.P is the phase voltage output from the stator winding and delivered to the load.
A phasor diagram of the circuit of FIG. 1 is shown in FIG. 3 with the power angle represented by .theta.. In a "passive" diode bridge (i.e., a rectifier bridge where only diodes are used) such as that shown in FIG. 1, the phase current and phase voltage are forced to be in the same phase relation. FIG. 3 demonstrates that when the phase voltage and phase current are forced to be in the same phase relation, it is not possible to achieve an optimal power angle such that the back emf and the phase voltage are orthogonal. However, if the passive diode bridge is replaced by a "controlled" or "active" transistor bridge (i.e., a rectifier bridge where the diodes are replaced by transistor switches), the phase voltage may be allowed to lag the phase current. A phasor diagram of an alternator having an active transistor bridge is shown in FIG. 3a. As can be seen from FIG. 3a, when the phase voltage is allowed to lag the phase current, the phase angle between the back EMF and the phase voltage (i.e., .theta.) may approach the optimal 90.degree. mark. It has been demonstrated that the power output of an alternator at idle speeds can be increased by 45 to 50% by optimizing the power angle toward the 90.degree. mark by advancing the phase angle of the phase voltage. For example, a 25.degree. phase advance in the phase voltage may result in a 45% increase in generator output.
Thus, in order to control the power angle, a "controlled" rectifier bridge is used in place of a "passive" rectifier bridge. In a controlled rectifier bridge, the angle of the phase voltage can be controlled by turning the transistor switches on and off at selected times. If the angle of the back EMF is known, the angle of the phase voltage may be adjusted by the switches in the controlled rectifier bridge and a more optimal power angle may be introduced to the alternator. Unfortunately, it is difficult to obtain a direct reading for the back EMF generated in the stator windings. Without a machine reference for the back EMF, the optimal phase angle for the phase voltages can not be determined. Therefore, some method must be used to obtain an indication of the phase angle of the back EMF before the phase voltage is shifted by the controlled bridge rectifier.
Several methods for adjusting the power angle are disclosed in U.S. Pat. No. 5,793,167 to Liang et al. Many of these methods require sensors. In one method, a rotor position sensor is used to provide a back EMF reference. In another method, a current sensor is used to determine the phase current, and adjustment of the angle between the phase current and phase voltage affects the power angle. However, there are several disadvantages to any method that relies on sensors for establishing a machine reference for use in adjusting the power angle. In particular, sensors are often unreliable, add substantial costs to the alternator, and make the alternator larger and more difficult to package. Therefore, it would be advantageous to provide a simple and easy to implement sensorless method of determining a machine reference for use in controlling the power angle in an alternator.